Dimethyl carbonate (DMC) is an important intermediate and is widely used in industry. Owing to its low toxicity, dimethyl carbonate is considered a “green” chemical product with bright development prospects. DMC is a versatile chemical and has been used mainly as a methylating and methoxy carbonylating agent as a safe substitute for dimethyl sulphate, phosgene or methyl halide, which are toxic or corrosive. It can also be used as a solvent to replace halogenated solvents. DMC has a high octane number and could be a good additive for gasoline in the future and could lead to increase in demand of DMC. This increasing focus on the use of DMC is mainly due to the bio-degradability, with a low bioaccumulation as well as its low toxicity.
Presently, DMC is produced mainly from methanol and phosgene in concentrated NaOH solution. Because of the use of phosgene for its production, DMC has been limited in industrial use. DMC also can be produced by non-phosgene route which includes oxidative carbonylation of methanol in liquid phase, which is put on stream in the EniChem Ravenna factory using CuCl catalyst. The major drawbacks of this process are low production rate; high cost of separation of products and reactants, high recycle requirements and the need for corrosion resistant reactor and process lines. Another non-phosgene process is the synthesis of DMC by transesterification of cyclic carbonate with methanol developed by Asahi Kasei Chemical, Japan. The main disadvantages of the route are: the slow reaction rate of epoxides with CO2 and requirement of high pressures, and the exchange reaction of the cyclic carbonate with methanol are limited by equilibrium.
The economy of the process is affected due to the use of epoxide which is expensive and formation of ethylene glycol as a by-product in stoichiometric quantity.
CO2 as a readily available, inexpensive and environmentally acceptable material has been widely investigated as raw material for DMC synthesis. But still the progress made so far is not satisfactory due to the difficulty of activation of CO2, and thermodynamic limitations.
Synthesis of DMC by the reaction of urea and methanol is an attractive alternative route. This will be a GREEN Process, being based on cheap and renewable raw materials.
The reaction scheme is presented below:

There are several patents as well as publications in recent times on the synthesis of DMC from methanol and urea as enumerated below:
U.S. Pat. No. 4,436,668 relates to preparation of carbonates of formula I,
by reacting a carbamic acid ester of the formula II;
with an alcohol of the formula R1—OH (wherein R1 and R2 have the meanings as described in said patent), in presence of stripping gas and in presence or absence of a catalyst. The final product is removed from the mixture by fractional distillation. This patent does not talk about mode of operation for the reactor used (batch or continuous) and also range of carbonates covered is higher. For higher dialkyl carbonates products are nonvolatile and only ammonia is removed during inert gas stripping. The details regarding stripping of product carbonates along with ammonia are not elaborated in the patent. Thus it is not clear how the stripping is carried out. This is crucial, since product dialkyl carbonates are also not stable under reaction conditions and can undergo decarboxylation reaction.
An article titled “High-Yield Synthesis of Dimethyl Carbonate from Urea and Methanol Using a Catalytic Distillation Process” by Mouhua Wang, Hui Wang et. al. in Ind. Eng. Chem. Res. 2007, 46, 2683-2687 relates to catalytic distillation technique for DMC synthesis which minimizes other side reactions. The article further discloses DMC yield of 60-70% by catalytic distillation reactor over a Zn-based catalyst. In this method DMC formed as a product is removed from the reaction as it is formed and the distillation condenser has a reflux ratio adjusted. Because of this product DMC is removed from the reaction mixture and part of the DMC product along with solvent methanol is returned back to the reactor. DMC returning to the reaction mixture can further react to form MMC as by-product or decompose to form DME, thus reducing overall yield of the DMC.
An article titled “Synthesis of dimethyl carbonate from urea and methanol using polyphosphoric acid as catalyst” by Jianjun Sun et. al in Journal of molecular catalysis A. chemical 239 (2005) 82-86 disclose synthesis of DMC from urea and methanol in a batch operation. Polyphosphoric acid (PPA) is used as catalyst and absorbent for ammonia produced during the reaction. Drawback of this approach is that the phosphoric acid reacts with ammonia formed and is consumed during the reaction.
It is obvious that for any reversible reaction, removal of at least one of the products from the reacting medium will drive the reaction in a forward direction. However, most of the prior art emphasizes on stripping ammonia from reactant mass and not on removal of DMC from the reactant mass. The inert gases used in the prior art include N2, CO2, Argon, He, ethane, methane and propane. However, none of the prior art disclose stripping with superheated vapors which have several advantages over previously disclosed stripping agents.
In some existing prior arts that disclose use of catalytic distillation, stripping may occur within the portion of catalytic distillation column. In these cases reflux ratio is fixed and this can lead to part of DMC product returning back to the reactor. DMC is known to decompose in the presence of catalysts and this can lead to loss in the yield of DMC. However, this stripping is with saturated methanol and in counter-current fashion which happens in any distillation column with stripping section. Further, there is no prior disclosure on any other form of stripping.
The inventors of the instant invention observed that through efficient removal of by-product, ammonia is necessary to shift the equilibrium in forward direction, simultaneous effective removal of DMC can reduce decomposition of DMC thereby enhancing the yield and selectivity. But prior arts do not provide effective means to perform the same. To overcome this gap in the art, the inventors have realized the removal of DMC by using packed bed reactor and sectionalized bubble column reactor.
Further, while packed bed reactor and bubble column reactors are known in the art, it is however, desired to provide a bubble column reactor with improved configuration that can allow significantly better stripping than that of packed column (counter-current stripping) or stirred reactors or vertical bubble column reactors. It is further desirable to provide horizontal sectionalized bubble column reactor with one or many sections configured to allow stripping of both ammonia and DMC simultaneously, which is not known in the art.